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Abstract

Star formation is fueled by dense molecular gas residing in the dense and cold in- teriors of molecular clouds. For decades, the emission of selected molecules, such as HCN and HCO+, has been used to trace this dense molecular gas phase within the Milky Way and external galaxies. Studies have revealed a tight link between star for- mation rate and the emission of these molecules, observed on both sub-cloud scales in the Milky Way and at kiloparsec scales in other galaxies. However, it has been recently established that the properties of molecular clouds significantly depend on the galactic environment they are residing in. Yet, observations of particularly the dense molecular phase – the direct fuel of star formation that resides within those clouds – are often limited to kiloparsec-scale resolution insufficient to access cloud properties or resolved studies of molecular clouds in a single environment only. To understand how exactly the large-scale galactic environments within galaxies are able to regulate the properties of star formation within molecular clouds, it is essential to gain a cloud-scale view of dense gas across a range of galactic environments. This thesis provides the first piece in understanding this link, with the most compre- hensive analysis of common dense gas tracers on cloud-scales across a diverse set of galactic environments. “Surveying the Whirlpool galaxy at Arcseconds with NOEMA” (SWAN), is the largest cloud-scale mapping of 3-4 mm emission in an external galaxy to date and the focus of this thesis. The detailed view provided by SWAN shows that dense gas is not only confined to spiral arms, but that its tracers emit brightly in the interarm region and galaxy center, suggesting future star formation across the disk. The first time in-depth comparison between common extragalactic dense gas tracers and the Galactic ‘gold standard’ tracer N2H+ reveals significant variations with both large-scale environment and local cloud- scale regions, which are undetected at coarser kiloparsec-scale resolution. Particularly, the center of M51 emerges as an extreme environment in which the relation between these dense gas tracers varies strongly. The utility of HCN emission in tracing gas den- sity, breaks down when tested in M51’s (extreme) environments and HCO+ presents itself as a more robust dense gas tracer. While studies from Milky Way clouds have questioned the use of HCN in tracing high gas density regions, this is the first time it has been tested at cloud-scales for entire cloud populations and the first time this could be placed into the context of galactic environments. Our new insights on the physical conditions that drive the dense gas emission reveal that gas density is not the sole driver of their emission. This implies that second order dependencies on other physical parameters like dynamical equilibrium pressure, star-formation rate and stel- lar mass surface density have a pronounced effect and cannot be neglected in further analysis. This thesis demonstrates the importance to place the star formation laws into an environmental context.